Small relays and switches, known as microswitches, are used in a wide variety of applications, such as automotive, test equipment, switchboard and telecommunications applications, for a wide variety of purposes. Due to the general trend of miniaturization of electronics, there is a growing need for ever-smaller relays and switches. Thus, a great deal of research has been conducted in the miniaturization of traditional electromechanical switches. However, traditional electromechanical switches, such as that switch shown in FIG. 1, tended to be inherently unreliable due to their mechanical nature—at some point the moving parts and/or the physical electrical contacts within the switches failed.
Referring to FIG. 1, illustrative prior art electromechanical switch 100 has load contacts 101a and 102a connected to arms 101 and 102, respectively. When an input voltage is applied to coil mechanism 111, the input voltage magnetizes the core 110, which pulls arm 106 and contact 108 toward contact 109 of core 110 in direction 107. This action causes, in turn, contact 103 on arm 105 to move upward in direction 104, thus physically pushing arm 102 upward in a way such that the load contacts 101a and 102a come into contact with each other, thereby closing the load circuit. When the input voltage is removed from coil 111, the spring lever 113 functions to push the contacts 101a and 102a away from each other, thus breaking the load circuit connection.
As discussed above, switch 100 must inherently make mechanical contacts in order to switch a load. At the point of these contacts, oxidation breakdown occurs over time at which point the switch will require maintenance or replacement. When switch 100 is activated, there is a window of time during which the load circuit flickers between an open and a closed state. This phenomenon, referred to as “bounce”, creates a delay between the time current is applied and the time the circuit is switched, thus creating a condition which may need to be considered in load design.
More recently, research efforts have focused on the development of miniature Microelectromechanical Systems (MEMS) switches which are, for example, fabricated using integrated circuit manufacturing processes, typically resulting in a lower production cost. However such switches are still somewhat unreliable because MEMS switches, similar to the electromechanical switch 100 of FIG. 1, typically are characterized by solid contacts that must be brought into contact with each other to close the switch. Accordingly, while MEMS switches are less costly than larger scale electromechanical switches, the aforementioned problems with electromechanical switches remain. Specifically, the solid-to-solid contact surfaces required in such switches suffer from the same oxidation/degradation during normal cycling. At relatively smaller scales, this can result in increased contact resistance, stiction, or microwelding at high throughput power.
In order to prevent the aforementioned problems associated with small electromechnical switches, other efforts have focused on solid state relays (SSRs). FIG. 2 shows a typical prior art Metal Oxide Semiconductor Field-Effect Transistor (MOSFET)-based SSR 200. Specifically, an input current is applied to light-emitting diode 201, which is illustratively a Gallium Arsenide (GaAs) infrared LED, via leads 202 and 203. The resulting emitted light 204 is reflected within an optical dome 205 onto a series of photo diodes 206 in photodiode array 206a. The photodiodes generate a voltage which is passed to driver circuitry 207 which, as is well known, is used to control the gates of two MOSFETs 208 and, accordingly, to switch a load 209.
All of the components of SSR 200 are, for example, fabricated out of semiconductor material and as a result, the solid state relay combines many operational characteristics not found in other types of devices. Because there are no moving parts, in contrast to the aformentioned electromechanical switches, solid state relays are characterized by relatively long switching lives and exhibit bounce-free operation. Additionally, the input LEDs require low signal levels, thus making such SSR switches attractive in low power applications.
While SSRs are, for the above reasons, advantageous in many applications, they are also limited in their usefulness in certain respects. Specifically, such devices may experience a relatively high degree of current leakage and, thus, may be relatively inefficient. Also, SSRs are typically limited in the power they can carry and, accordingly, are likewise limited in the ability to switch higher loads. Finally, SSRs are relatively expensive to manufacture compared to electromechanical and/or MEMs-based devices, thus increasing the cost of devices that use SSRs.
In order to address some of the above limitations, recent attempts have focused on developing liquid based microswitches. Such attempts are described in, for example, J. Kim et al., “A micromechanical switch with electrostatically driven liquid-metal droplet,”, SENSOR ACTUAT A-PHYS 97–8: 672–679 Apr. 1, 2002; L. Latorre et al., “Electrostatic actuation of microscale liquid-metal droplets,” JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 11, NO. 4, AUGUST 2002 and J. Simon, S. Saffer, and C.-J. Kim, “A Liquid-Filled Microrelay with a Moving Mercury Micro-Drop”, J. Microelectro-mechanical Systems, Vol. 6, No. 3, September 1997, pp. 208–216, which are hereby incorporated by reference herein in their entirety. As described in these references, typical prior liquid switches use a movable liquid droplet that either comes into contact or separates from the solid electrodes, thus connecting or disconnecting electrical circuit. Such an approach is limited in that the wetting and electrochemical phenomena at the liquid-solid contact interface can be relatively unreliable.